Polymer-based stimuli responsive polymers and materials have been of considerable interest for many years due to their ability to convert a chemical or physical stimulus into an observable change in a system. Hydrogel-based thin films, assemblies, and particles (microgels and nanogels) have been designed to respond to a variety of stimuli, for a number of potential application in tissue engineering, artificial muscles, valves, and actuators.[1-9] Hydrogels are of particular interest due to their mechanical properties, chemical diversity, and hydration properties, which allows them to interface well with biological systems. Recently, responsive hydrogels and polymer-based thin films has been developed as programmable soft matter or motors by exploiting conformational changes of the polymer that effects the system.[4,6] Of specific interest to the investigation here are responsive polymer-based systems that are able to do work, i.e., lift a mass.[10-12] These systems, often referred to as artificial muscles, have been the subject of intense research due to their potential to control movements in mechanical motors.[10-13]. One of the most well studied responsive polymers to date is poly (N-isopropylacrylamide) (pNIPAm) which shows random coil to globule transition at temperatures below ˜32° C.[14,15]
Charged pNIPAm-based microgels have been synthesized and used for various applications.[16-18] By far, the most common chemical functionality added to pNIPAm-based microgels is acrylic acid (AAc). AAc is a weak acid, having a pKa of ˜4.25, therefore at pH>4.25 the AAc groups are deprotonated making the microgels negatively charged (polyanionic), while they are neutral at pH<4.25 due to AAc protonation. At high pH the microgels swell due to the charge-charge (Coulombic) repulsion in the microgel's polymer network.
Materials that spontaneously undergo a change in structure, e.g., from a two dimensional (2D) to three dimensional (3D) structure in response to external stimuli have been of great interest as artificial muscles, and for fabricating novel actuators, switches, valves and in robotics.S1, S2, S3, S4, S5, S6 Various stimuli responsive polymers have been identified, which exhibit responses to electric fields, temperature, light, pH, ionic strength, humidity and/or solvent composition.S7, S8, S9, S10, S11 Recently, responsive hydrogels and polymer-based films have been used as materials capable of converting chemical or physical energy into mechanical forces, which can lead to macroscopic changes to a material's conformation.S12 Specifically, temperature responsive poly (N-isopropylacrylamide) (pNIPAm) based hydrogel sheets capable of transforming from a planar state to a 3D structure have been developed by tuning the concentration of monomers and crosslinking density in the hydrogel sheets.S13 Regions with different polymer content went through differential deformation upon heating allowing the formation of unique 3D structures. Wang et al. recently fabricated near-infrared light-driven hydrogel actuators by interfacing reduced-graphene oxide sheets with protein-based polymers. These hydrogels showed rapid, reversible bending motion at specific positions where near-infrared laser was applied.S14 Another system composed of a soft poly (butadiene) phase and a hard metal-ligand phase was developed, which exhibited shape-memory properties in response to external stimuli.S15 In this case, the key component in fulfilling the shape change is the metal-ligand phase which can become soft when exposed to a variety of stimuli (e.g., light, heat, chemicals).
Of recent, several types of humidity responsive polymers have also been used to fabricate actuators. For example, Langer and coworkersS16 recently developed a polymer composite of rigid polypyrrole (PPy) embedded with a flexible polyol-borate network. PPy can absorb water and change its shape, while the soft polyol-borate network is also sensitive to water, undergoing hydrolysis and reformation of the borate ester crosslinker upon water absorption and desorption, respectively. By breaking and reforming intermolecular hydrogen bonding between PPy and the polyol-borate network and the borate ester within the polyol-borate network upon water sorption and desorption, the film shows expansion and contraction, resulting in the film's rapid and continuous locomotion. In another example, Sun and coworkers developed one bilayer film consisting of a polyelectrolyte multilayer (PEM) film and a layer of UV-cured prepolymer.S17 The PEM is a film of thermally crosslinked poly (acrylic acid) (PAA)/poly (allylamine hydrochloride) (PAH). The PAA/PAH is able to absorb/desorb water with increasing/decreasing environmental humidity, which resulted in swelling/shrinking of the layer. They fabricated an energetic walking device driven by the powerful humidity responsive PEM.